Which value must be added to the expression x2 + 12x to make it a perfect-square trinomial?

For an expression of the form x² + bx to become a perfect-square trinomial, add the square of half the coefficient of x. Half of 12 is 6, and 6² = 36, giving x² + 12x + 36 = (x + 6)². The correct choice is 36.

AI-written and checked against the verified answer.